1. Field of Invention
This invention is directed to iron-based mixed powders for use in metallurgy.
2. Description of Related Art
Iron-based mixed powders for use in powder metallurgy hereinafter also referred to simply as xe2x80x9ciron-based mixed powderxe2x80x9d) are manufactured, generally, by adding: (1) an iron powder for an iron-based powder as a substrate material (which can be a mixture of one or more kinds of iron powder), (2) alloying powder(s) (one or more kinds of alloying powder such as a copper powder, graphite powder and iron phosphide powder), optionally, (3) a lubricant such as zinc stearate (which can be a mixture of one or more kinds of lubricant) and, optionally, (4) machinability improving powder(s) (one or more kinds of machinability improving powder).
However, the iron-based mixed powders described above have a problem that the starting powder, particularly, the alloying powder(s) tends to cause segregation. This is because the iron-based mixed powder contains plural kinds of powder of different sizes, shape and density. Specifically, the distribution of starting powders in the iron-based mixed powder is not uniform during transportation after mixing, charging to a hopper, discharging from the hopper, or upon charging to the mold or during pressing.
For example, it is well-known for the mixed powder of the iron powder and the graphite powder that the iron powder and the graphite powder move and displace independently of each other in a transportation container during track transportation and, as a result, the graphite powder of lower specific gravity floats to the surface and causes segregation. Further, because the mixed powder of the iron powder and the graphite powder charged in the hopper segregates due to movement in the hopper, it is also well-known that the concentration of the graphite powder is different, for example, between each of the initial stage, the middle stage and the final stage of discharging from the hopper.
When the segregated iron-based mixed powder is charged in a mold and pressed into a molding product and the molding product is finally sintered into a sintered body as a final product, the composition fluctuates for every product (sintered product). As a result of the fluctuation of the composition, the size and the strength of products vary greatly to cause failed products.
Further, because each of the alloying powders to be mixed, such as copper powder, graphite powder and iron phosphide powder, is finer than the iron-based powder, the specific surface area of the iron-based mixed powder increases by the mixing of the alloying powder(s) to lower the fluidity of the iron-based mixed powder. Lowering the fluidity of the iron-based mixed powder lowers the charging rate of the iron-based mixed powder into the mold and, therefore, lowers the production speed of the molding product (also referred to as compact powder or green compact).
As a countermeasure for such problems in iron-based mixed powders, as a technique of preventing segregation, Japanese Patent Laid-Open No. 219101/1989, for example, proposes an iron powder for use in powder metallurgy, comprising from 0.3 to 1.3% of a lubricant, from 0.1 to 10% of an alloying element powder and the balance of an iron powder, in which the alloying element powder is adhered on the surface of the iron powder. According to this publication, the iron powder causes no segregation of the ingredients during handling and enables to obtain homogeneous sintered products.
Further, Japanese Patent Laid-Open 162502/1991 discloses a method of manufacturing an iron-based mixed powder for use in powder metallurgy with less segregation of additives and less aging change of the fluidity. The method described in Japanese Patent Laid-Open No. 162502/1991 comprises conducting primary mixing by adding a fatty acid to an iron-based powder, then conducting secondary mixing by adding a metal soap to the alloying powder(s), elevating the temperature during or after the secondary mixing, and then applying cooling during tertiary mixing, thereby adhering the alloying powder(s) to the surface of the iron-based powder by a binding effect of a co-molten product of the fatty acid and the metal soap.
Japanese Patent Publication No. 3004800 discloses an iron-based mixed powder using a binder not containing a metal compound as a binder for the alloying powder(s) to the surface of the iron-based powder. It is described that contamination to a sintering furnace can be reduced by the use of the binder material not containing the metal compound.
However, the iron-based mixed powder applied with the segregation-preventive treatment by each of the publications described above has a problem in the die filling property to a mold and, particularly, has a property that the amount of charge to a narrow width portion of the mold (thin-walled cavity) tends to be decreased.
In view of the above, the present inventors have experimentally confirmed the die filling property of the iron-based mixed powder applied with the segregation-preventive treatment disclosed by the publications described above. First, the result of this experiment is explained as follows.
To an atomized iron powder as the iron-based powder, 2 mass % of a copper powder and 0.8 mass % of a graphite powder as the alloying powder(s), and 0.4 parts by weight of zinc stearate and 0.2 parts by weight of machine oil (spindle oil) as the binder based on 100 parts by weight of the total sum of the iron power and the alloying power, were mixed and heated to adhere the alloying powder(s) to the surface of the iron powder (example of a binder treatment). Then, 0.3 parts by weight of zinc stearate was mixed with these components as a free lubricant. An iron-based mixed powder including a mixture of an iron powder and a free lubricant, in which alloying powder(s) is adhered on the surface of the iron powder (known product), was obtained by this treatment. 150 g of the iron-based mixed powder was charged in a shoe box sized 20 mmxc3x9760 mmxc3x97100 mm, as shown in FIG. 1.
The shoe box was moved in a direction to a mold at a speed of 200 mm/s, stood stationary just above the mold for 1 second, and then retracted to the original position in the arrangement, as shown in FIG. 1. The iron-based mixed powder was charged into the mold by the operation. The mold used has a cavity with a thickness of T mm, length, L, of 60 mm and depth, D, of 60 mm. The thickness T mm was varied as 1, 2 and 5 mm.
After charging, the iron-based mixed powder charged in the cavity was molded at a pressure of 488 MPa and the weight of the obtained molding product was measured. Then, the charged density (=the molding product weight/mold volume) was calculated to evaluate the die filling property of the iron-based mixed powder to the mold. The result for the iron-based mixed powder (known product) is shown in FIG. 2. It can be seen from FIG. 2 that the charged density decreases as the cavity thickness T of the mold decreases in the known product. For example, when the cavity thickness T of the mold is 1 mm, the existent iron-based mixed powder is charged by less than one-half for the apparent density. As described above, when the cavity thickness of the mold is thin, die filling property of the iron-based mixed powder treated for segregation by the known techniques is deteriorated.
In the known product of the reduced die filling property as described above, when it is charged into a mold, for example, of a gear shape, the charged density is lower at a narrow width portion of the tooth tip as compared with other portions of the gear. Then, when it is pressurized as it is into the molding product and further sintered, because the amount of shrinkage differs depending on the portions, the dimensional accuracy of a part is deteriorated. Generally, when the charged density is different and the green density is different for different portions, the rate of dimensional change upon sintering also differs and, further, the sintering density is also different. Accordingly, in the portion at the tooth tip of the gear of low charged density, the sintering density tends to be lowered and, thus, the strength is lowered. Because maximum stress is usually exerted on the portion of the tooth tip in the gear, it is required that the portion for the tooth tip has a higher strength and, preferably, the charged density is preferably higher.
In view of the problems described above, Japanese Patent Laid-Open No. 267195/1997 discloses, for example, a powder charging method comprising disposing a pipe having gas releasing holes at the surface in a shoe box, fluidizing a powder by the gas exiting from the gas releasing holes, and then charging the powder gravitationally into the cavity. However, because the technique described in Japanese Patent Laid-Open No. 267195/1997 requires a special apparatus, it has a problem of increasing the installation cost and also increasing the manufacturing cost.
Further, in the field of sintered parts for use in automobiles, for instance, reduction of size for sintered parts is desired along with a demand for the weight reduction of car bodies in recent years. However, stress exerted on parts tends to be increased along with the size reduction of the parts. Accordingly, for the parts of identical composition, those parts of higher strength, namely, those parts of higher density are desired (for sintered products of an identical composition, the strength is generally higher as the density is higher). In order to obtain a sintered part of a reduced size and having high density, it is necessary that the iron-based mixed powder is applied with the segregation-preventive treatment and is excellent in compressibility. In addition, it is required for an iron-based mixed powder that it is excellent in the die filling property to the narrower width portion of the mold, as well as it having the characteristics described above.
This invention can advantageously overcome the problems of known powders described above and provide an iron-based mixed powder capable of manufacturing sintered parts of consistent high density and with less fluctuation of characteristics. Specifically, it intends to provide an iron-based mixed powder applied with a segregation-preventive treatment and excellent in the compressibility (high density for the molding product) and excellent in the die filling property.
The present inventors have made an earnest study in order to solve the foregoing problems of various factors affecting the compressibility and the die filling property of the iron-based mixed powder applied with the segregation-preventive treatment (for example, a binder treatment).
First, the iron-based powder is generally classified into two types of powder; namely, an atomized iron powder and a reduced iron powder. The reduced iron powder has greater unevenness on the surface and more voids in the iron powder as compared with the atomized iron powder. Accordingly, it is well-known that iron-based mixed powder using reduced iron powder has lower compressibility and poor fluidity (flow rate) compared with those using atomized iron powder. While the fluidity and the die filling property are not an identical property, it can be generally anticipated that good fluidity will be advantageous for die filling property. Further, the iron-based mixed powder of excellent fluidity can be industrially handled more easily.
Accordingly, for obtaining high sintered density required generally for sintered parts, atomized iron powders excellent in compressibility and fluidity of the mixed powder have usually been used as the iron-based powders (reduced iron powder may exceptionally be used in bearing parts in order to utilize the oil-preserving effect of voids).
As a result of the study, the present inventors have found that the iron-based mixed powder using reduced iron powder is more excellent than iron-based mixed powder using atomized iron powder with respect to the die filling property to the mold having a narrow cavity, contrary to the analogy from the fluidity.
On the other hand, it is difficult to obtain a sufficient compressibility in iron-based mixed powder using reduced iron powder as the iron-based powder. The present inventors have made a further study and discovered that the die filling property of the iron-based mixed powder can be improved remarkably with no significant lowering of the compressibility by mixing an appropriate amount of reduced iron powder to atomized iron powder as a main component. The present inventors have further found that use of an appropriate binder and a lubricant can also further improve the die filling property.
An example of the die filling property of the iron-based mixed powder according to this invention is shown in FIG. 2 as the inventive product. The iron-based mixed powder according to this invention (inventive product) can be charged well even for a cavity thickness of 1 mm, and it can be seen that the die filling property is remarkably improved compared with the known product.
This invention has been accomplished based on the findings described above and as a result of a further study.
That is, this invention provides an iron-based mixed powder for use in powder metallurgy that has excellent die filling property, comprising an iron-based powder, alloying powder(s), a binder and, optionally, a machinability improving powder(s) and, preferably, further containing a free lubricant. The iron-based powder comprises from about 60% to about 90% of an atomized iron powder and from about 10% to about 40% of a reduced iron powder on a mass % basis, based on the entire amount of the iron-based powder (preferably, the balance excepting the atomized iron powder substantially comprising the reduced iron powder), and the alloying powder(s) and, optionally, the machinability improving powder(s) are adhered by the binder to the surface of the iron-based powder.
Further, in the invention described above, it is preferred that the reduced iron powder used for the iron-based powder is present as a free iron-based powder (iron-based powder with no alloying powder or the machinability improving powder adhered on the surface) in an amount of from about 10% to about 30% for the entire amount of the iron-based powder. For this purpose, the free iron-based powder may be mixed after the binder treatment.
Further, in the invention, the content of the binder is preferably from about 0.1 parts by weight to about 1.0 parts by weight based on 100% by weight of the total amount for the iron-based powder, alloying powder(s) and the machinability improving powder(s).
Further, in this invention, the binder is preferably one or more members selected from stearic acid, oleamide, stearamide, a melted mixture of stearamide and ethylenbis(stearamide) and ethylenbis(stearamide).
Further, in this invention, the binder may comprise one or more of members selected from oleic acid, spindle oil and turbine oil, and zinc stearate.
Further, in this invention, the content of the free lubricant is preferably from about 0.1 parts to about 0.8 parts by weight based 100 parts by weight of the total amount for the iron-based powder, the alloying powder(s) and the machinability improving powder(s).
Furthermore, in this invention, the free lubricant preferably comprises one or more members selected from a thermoplastic resin powder, zinc stearate and lithium stearate, or, optionally, contains one or more members selected from stearic acid, oleamide, stearamide, a melted mixture of stearamide and ethylenbis(stearamide), ethylenbis(stearamide), polyethylene with a molecular weight of about 10,000 or less, and a melted mixture of ethylenbis(stearamide) and polyethylene with a molecular weight of about 10,000 or less.
Further in this invention, the thermoplastic resin powder preferably comprises 50 mass % or more, based on the thermoplastic powder, of at least one member selected from acrylic esters, methacrylic esters and the aromatic vinyl compounds as a monomer polymerized therewith, and has a average primary particle size of from about 0.03 xcexcm to about 5.0 xcexcm, an average agglomeration particle size of from about 5 xcexcm to about 50 xcexcm, and an average molecular weight, measured by a solution specific viscosity method, of from about 30,000 to about 5,000,000.